Cryogenic-Assisted Hydrogen Fluoride Surface Reactions Enabling Reversibly Ultra-High Selectivity of Atomic Layer Etching Between SiO2 and SiN.

Atomic-level precision processes are increasingly essential for advanced semiconductor devices with highly-complicated small features. Plasma-enhanced atomic layer etching (ALE) is considered as a promising technique to meet requirements of material diversity and highly selective processing. Here, the ALE processes of SiO2 and SiN are demonstrated through manipulating hydrogen-fluoride (HF) reactions for surface modification, followed by argon ion bombardment for material removal. By varying substrate temperature (Ts) and introducing ethanol (C2H5OH) gas, which provides hydroxyl groups for cryogenic-assisted synergistic reactions, the surface HF reactions and the properties of the surface modification layer can be significantly influenced. The etch amount per cycle (EPC) of SiN ALE decreases to zero with decreasing Ts, regardless of C2H5OH addition, due to increased stability of the (NH4)2SiF6 modification layer. No ALE synergy is observed for SiO2 when C2H5OH is not added during the HF dose step, irrespective of Ts. Conversely, the addition of C2H5OH at cryogenic temperatures enables the synergistic interactions between HF molecules and hydroxyl groups, enhancing the co-adsorption of HF/C2H5OH and lowering activation energy for the fluorination reaction of the SiO2 that leads to the increased EPC. Consequently, the reversible etching selectivity between ALE SiO2 and SiN, reaching up to infinity, is achieved.